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Coral Competition

Turf Wars in Coral Reef Tanks

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Sweeper Tentacles

Sweeper Tentacles

Lucia Gavin

In the wild, coral reefs are in a constant slow motion turf war. Both hard and soft corals have the ability (barring outside influences) to cover up to 85% of the substrate on a reef, being quite successful at defeating invading organisms, such as algae. This leaves the battle for a majority of the reef real estate to the corals, which deploy both offensive and defensive weapons in order to survive and propagate.

The same battle for turf can be going on in your reef tank, too. A majority of the elements involved in reef survival are concentrated in a reef aquarium. By planning coral placements in your tank and being aware of developing turf conflicts, a majority of problems can avoided or controlled.

Coral Toxicity to Fish shows the potential affects of coral toxins on fish while Signs of Coral Competition shows what the indications are that your corals may be competing for turf in your tank.

Corals acquire and maintain space on a reef by:

    • Reproducing rapidly.
    • Competing for available food.
    • Having offensive and defensive weapons.
    • Having the ability to sustain damage and still continue to reproduce.
There is a price for doing battle on the reef: The energy spent on both offense and defense uses precious resources that could otherwise be spent on growth and reproduction.

It has been estimated that, on the reef, between 22% and 38% of all coral colonies are engaged in battle or are within range to engage. Fortunately, more corals are passive than aggressive. Unfortunately, a good defense does not always win. Most corals must be aggressive in order to survive. Four levels of engagement have been proposed (Rinkevich and Loya, 1985):

Types of Behavioral Competition

Extracoelenteric Digestion:

  • Corals expel digestive filaments which contain cnidocytes (digestive fluids).
  • Cnidocytes can be expelled from the digestive track en masse (puking) onto a nearby coral, digesting it.

Sweeper Tentacles:

  • Much longer than other ("normal") tentacles.
  • "Sniff" the water for invaders.
  • Only form in numbers when chemical contacts from other coral colonies are made.
  • Capable of feeding, but used primarily as weapons.
  • The tips break off and stick to other corals when contacted.
  • After contact, they continue to discharge nematocysts, damaging the invader.
  • Reduced or no water movement in a reef tank will usually cause the sweepers to retract.
  • Euphyllia and Galaxea species are known for forming very long, potent tentacles.
You can see a terrific example of Sweeper Tentacles in the photo to the right. Click Here for an Enlarged View

Stinging Nematocysts

  • Used on short tentacles for close range offense and defense.
  • Can be fired long range, stinging any corals downstream.
  • Can sting humans.

Acrorhagi

  • Specialized cnidocytes found on sweeper and column projections with concentrated nematocysts.
  • Require close contact with other corals to be effective.
  • Normally result in the death of contacted tissue.
  • A specialized defense only used in response to same species corals and certain cnidarians that create a response.

Mucus

  • Can be toxic or contain nematocycts.
  • Can carry long distances in the water current.
  • Can be quite damaging as it "sticks" to corals.
Morphological Competition:

Overgrowth

  • Dominate corals can grow right over other corals.
  • Commonly used by octocorals and stoloniferans.

Oriented Translocation

  • The ability of fused (joined) corals to extract nutrition from the overrun coral.

Overtopping

  • Fast growing corals can grow over slower growing corals, blocking their sunlight.
  • May not always cause death of the shadowed coral.

Movements

  • Some corals have the ability to move about on the reef.
  • Some corals can detach from the substrate.
  • Corals use tentacular and mucosal contact for defense against encountered competitors.
Chemical Competition:
  • The production of toxic compounds is known as allelopathy.
  • Most commonly known producers of toxins are soft corals and gorgoneans.
  • Toxins emitted by these corals can be lethal to fish.
  • Effective in the aggressive competition for space on the reef.
  • Excellent defense against predation and parasitism.

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